Golf balls generally include a spherical outer surface with a plurality of dimples formed thereon. Conventional dimples are circular depressions that act to reduce drag and increase lift. These dimples are formed where a dimple wall slopes away from the outer surface of the ball forming the depression. The circumference of each dimple is the edge formed sphere the dimple wall slopes away from the outer surface.
Drag is the air resistance that acts on the golf ball in the direction opposite the ball's flight direction. As the ball travels through the air, the air that surrounds the ball has different velocities thus, different pressures. The air exerts maximum pressure at a stagnation point on the front of the ball. The air then flows around the surface of the ball with an increased velocity and reduced pressure. At some separation point, the air separates from the surface of the ball and generates a large turbulent flow area behind the ball. This flow area, which is called the wake, has low pressure. The difference between the high pressure in front of the ball and the low pressure behind the ball slows the ball down. This is the primary source of drag for golf balls.
The dimples on the golf ball cause a thin boundary layer of air adjacent to the ball's outer surface to flow in a turbulent manner. Thus, the thin boundary layer is called a turbulent boundary layer. The turbulence energizes the boundary layer and helps move the separation point further backward, so that the layer stays attached further along the ball's outer surface. As a result, there is a reduction in the area of the wake, an increase in the pressure behind the ball, and a substantial reduction in drag. It is the circumference of each dimple, where the dimple wall drops away from the outer surface of the ball, which actually creates the turbulence in the boundary layer.
Lift is an upward force on the ball that is created by a difference in pressure between the top of the ball and the bottom of the ball. This difference in pressure is created by a warp in the air flow that results from the ball's backspin. Due to the backspin, the top of the ball moves with the airflow, which delays the air separation point to a location further backward. Conversely, the bottom of the ball moves against the air flow, which moves the separation point forward. This asymmetrical separation creates an arch in the flow pattern that requires the air that flows over the top of the ball to move faster than the air that flows along the bottom of the ball. As a result, the air above the ball is at a lower pressure than the air underneath the ball. This pressure difference results in the overall force, called lift, which is exerted upwardly on the ball. The circumference of each dimple is critical in optimizing this flow phenomenon, as well.
By using dimples to decrease drag and increase lift, almost every golf ball manufacturer has increased their golf ball flight distances. In order to optimize ball performance, it is desirable to have a large number of dimples, hence a large amount of dimple circumference, which is evenly distributed around the ball. In arranging the dimples, an attempt is made to minimize the space between dimples, because such space does not improve aerodynamic performance of the ball. In practical terms, this usually translates into 300 to 500 circular dimples with a conventional sized dimple having a diameter that ranges from about 0.120 inches to about 0.180 inches. "Small" dimples in this application mean those with a diameter less than about 0.120 inches, and "large" dimples mean those with a diameter greater than about 0.180 inches.
When compared to one conventional size dimple, theoretically, an increased number of small dimples will create greater aerodynamic performance by increasing total dimple circumference. However, in reality small dimples are not always very effective in decreasing drag and increasing lift. This results at least in part from the susceptibility of small dimples to paint flooding. Paint flooding occurs when the paint coat on the golf ball fills the small dimples, and consequently decreases the dimple's aerodynamic effectiveness. On the other hand, a smaller number of large dimples also begin to lose effectiveness. This results from the circumference of one large dimple being less than that of a group of smaller dimples.
U.K. Patent No. 2 215 621 discloses a dimple for use in a uniform distribution over the spherical, outer surface of a golf ball so that the dimple pattern has an overall, identical configuration irrespective of the direction of motion of the ball. In one embodiment, at least one dimple has a circular cavity surrounded by an annular cavity. The radial distance between the circular cavity and the annular cavity is described as up to 0.039 inches (1.0 mm). A radial distance this large is undesirable, since it means a large amount of the golf ball's outer surface is not covered by aerodynamically effective dimples. One embodiment, as shown in FIG. 7 of this patent, describes the annular cavity as of small dimensions and configuration relative to the circular cavity and shows the radial distance as larger than the width of the annular cavity. The reference discloses that the width of the annular cavity is between 0.0039 inches (0.1 mm) and 0.079 inches (2 mm).
Most balls today have dimple patterns with many spaces between dimples or have filled in the spaces with large dimples or groupings of small dimples that do not create the optimal aerodynamic effect at average golf ball velocities. It is desirable to provide a type of dimple that increases aerodynamic effectiveness and either fills spaces in the dimple pattern or replaces small or large dimples used in the past.